1. Field of the Invention
The present invention relates to an excitation controller for stabilizing voltage in an electric power system.
2. Description of the Background Art
An example of a conventional excitation controller for controlling excitation of a synchronous machine connected to a power transmission system through a transformer is disclosed in Japanese Laid-open Patent Publication No. 2000-308397 (corresponding to U.S. Pat. No. 6,265,852). The excitation controller of the Publication detects a voltage VG at an output terminal of the synchronous machine and a reactive current IQ output from the synchronous machine, causes a voltage setter to set an output terminal target voltage VGref of the synchronous machine based on the reactive current IQ and a high-side target voltage VHref of the transformer such that a relationship expressed by VGref=VHref+Xt·IQ is satisfied (where Xt is the reactance of the transformer), and controls an excitation system of the synchronous machine based on a deviation of the detective output terminal voltage VG from the output terminal target voltage VGref of the synchronous machine.
More specifically, the aforementioned conventional excitation controller estimates a high-side voltage VH of the transformer from the output terminal voltage VG, the reactive current IQ of the synchronous machine and the reactance Xt of the transformer by using a relationship VH=VG−Xt·IQ, from which the output terminal voltage VG of the synchronous machine is expressed by the following equation:VG=VH+Xt·IQ  (1)
Then, the excitation controller sets the output terminal target voltage VGref as indicated by the following equation to compensate for a voltage drop occurring in the transformer from its high-side target voltage VHref:VGref=VHref+Xt·IQ  (2)
However, since the amount of a voltage change in the transformer varies also with phase angle variations Δ δ occurring in the transformer, the output terminal voltage VG of the synchronous machine is actually given by the following equation:VG=VH·cos Δδ+Xt·IQ  (3)which is different from the value given by equation (1).
It is therefore impossible to exactly set the output terminal target voltage VGref of the synchronous machine, because the aforementioned phase angle variations Δ δ are not taken into account in the output terminal target voltage VGref calculated by equation (2) above. This calculation error becomes more significant as the phase angle variation Δ δ in the transformer increases. For this reason, it has been difficult to keep the high-side voltage VH of the transformer, that is, the voltage applied to a transmission bus, at the target voltage VHref with high reliability.